If you just want to make the throw blocks bounce off of something, you don't actually need custom blocks. (Although the custom blocks do have more functionality
and less limitations, so they're worth considering)
In vanilla, there are 2 ways (as far as i'm aware, there may be more) of doing that, although each of them come with a few limitations:
1. Using a very steep slope. The very steep slope object/tiles can bounce most sprites back when hit (even sprites that wouldn't be bounced in other ways, such as sliding Blue Koopas). You need to use both the top and bottom tile of the slope for it to work. The limitation of this is that Mario can stand on the slope, so it won't work if you need the block to be non-solid.
2. The Flying Question Block sprite (sprites 83 and 84) can be triggered by a throw block, and if done so by the side, it'll bounce the throw block back. The limitation for this one is that the sprite is solid to Mario, just like the slope, and also that it moves, so it takes some effort to line up the setups properly.
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